How does Model Grid Resolution Influence Mixed-Phase Processes and UTLS Moisture Transport by a WCB?

Warm conveyor belts (WBCs) are large-scale ascending airstreams found in extratopical cyclones. They constitute a major source of upper tropospheric/lower stratospheric (UTLS) water vapor—a potent greenhouse gas—and hydrometeors, which can form cirrus clouds. Therefore, WCBs play an important role for Earth’s radiative budget. Additionally, WCBs transport large amounts of heat across latitudes and can influence large scale upper tropospheric circulation after their dissipation, underscoring their significance for Earth’s weather and climate.

Recent studies using high-resolution, convection-permitting simulations have shown that convection is a prominent feature of WCBs, with convective air parcels transporting significantly more hydrometeors into the UTLS than their slower-ascending counterparts. Furthermore, the cloud and precipitation development in convective air parcels is dominated by different processes than in slower ascending air. However, the global numerical weather prediction and climate models commonly used to assess the climatological and future impacts of WCBs operate at coarse grid resolutions (15–50 km) that are not convection-permitting, relying instead on convection parametrization schemes. The widely used Tiedtke-Bechtold convection parameterization scheme is designed to simulate heat, moisture, and momentum transport in convective systems but includes only basic representations of cloud microphysics. This raises the question of whether low-resolution simulations fail to accurately represent the transport of hydrometeors into the UTLS by WCBs when compared to high-resolution, convection-permitting simulations.

To address this, we analyze two simulations of the same WCB—one convection-permitting and one using convection parameterization—with a specific focus on vapor and hydrometeor transport. Our results show that the WCB in the high-resolution simulation transports substantially larger amounts of hydrometeors into the UTLS, while UTLS vapor conditions remain comparable between the two simulations. Microphysics processes also shift from liquid-dominated to frozen-dominated depending on the grid-scale.